Conductive coating

ABSTRACT

A process of applying a conductive coating to an article to provide electromagnetic and electrostatic energy shielding and grounding. The article is first coated with a sufficient amount of a metallic copper pigmented resin to provide a pigment particle to particle contact upon drying. A synergistic effect is obtained by then coating the copper pigment with a graphite pigmented resin. A sufficient amount of graphite pigmented resin is used to prevent oxidation of the copper pigment and enhance electrical conductivity.

United States Patent Burdette, Jr. et al. Sept. 2, 1975 [54] CONDUCTIVE COATING 3,247,478 4/1966 Craig 117/226 3,298,896 1/1967 Szegvari 117/216 [75] Inventors: Russell 3,779,807 12 1973 Taylor et a1. 117/217 Lexmgton, y Dav"! Downmg 3,783,021 1/1974 York 117/226 Dean; William Lunsford Mitchell, both of Austin, Tex.

Assignee: International Business Machines Corporation, Armonk, NY.

Filed: Apr. 26, 1974 Appl. No.: 464,419

[52] U.S. Cl 427/122; 427/123 [51] Int. Cl. B44D 1/18; B44D 1/14 [58] Field of Search 1. 117/227, 226, 218, 217, 1 17/216 [56] References Cited UNITED STATES PATENTS 2,134,870 11/1938 Fruth 117/216 2,781,277 2/1957 Dwyer....

3,006,785 10/1961 Canegallo 117/217 Primary ExaminerCameron K. Weiffenbach Attorney, Agent, or Firm.lames H. Barksdale, Jr.

[5 7 ABSTRACT A process of applying a conductive coating to an article to provide electromagnetic and electrostatic energy shielding and grounding. The article is first coated with a sufficient amount of a metallic copper pigmented resin to provide a pigment particle to particle contact upon drying. A synergistic effect is obtained by then coating the copper pigment with a graphite pigmented resin. A sufficient amount of graphite pigmented resin is used to prevent oxidation of the copper pigment and enhance electrical conductivity.

13 Claims, No Drawings CONDUCTIVE COATING BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates generally to electromagnetic shielding and electrostatic grounding, and more specifically to a conductive coating of both copper and graphite for an article to provide both such shielding and grounding.

2. Description of the Prior Art Shielding and grounding, per se, in terms of use of conductive materials relative to electromagnetic and electrostatic energy are well known in the prior art. Metallic foils, screen wire, vacuum metallizing, etc., have been used with varying degrees of success. When costs, manufacturability, effectiveness, geographic availability, etc., are considered though, a completely satisfactory solution for both shielding and grounding problems has not been forthcoming. Although various coatings are available which provide some desirable characteristics, other problems, such as short life expectancy, flaking, and marginal effectiveness, have been encountered. Further, adherence of a coating to an article such as a molded polymeric housing has presented a problem of varying degrees and long standing. Also, depending upon the polymeric material making up the housing, electromagnetic and electrostatic discharge problems can be critical. This is particularly so where the housing contains electronic components.

SUMMARY OF THE INVENTION A conductive coating is applied to an article for providing an electromagnetic energy shield, and electrostatic grounding, through essentially a two step coating process. The article is first coated with a sufficient amount of a metallic copper pigment dispersed in a thermoplastic resin binder to provide a pigment particle to particle contact upon drying. Thereafter, the copper pigment coating is coated with a sufficient amount of conductive graphite, made up of a graphite pigment dispersed in a resin, to prevent oxidation of the copper coating and enhance electrical conductivity.

DESCRIPTION OF THE PREFERRED EMBODIMENT The interior of a machine housing which is to include electrical and electronic apparatus is provided with a conductive coating for electromagnetic and electrostatic energy shielding and grounding purposes. The housing can be made up of any moldable polymeric material such as a modified polyphenylene oxide, polyolefin, polyamide, polyvinyl chloride, etc., capable of storing a static charge.

To a clean interior surface of the housing is first ap plied a metallic copper pigmented thermoplastic resin through conventional spray painting techniques related to heavily pigmented paints. This first coating is then allowed to dry, or is dryed through application of heat. Following drying, a conductive graphite coating made up of a graphite pigmented water-dispersed resin is applied to the first coating. This second coating will elimi nate oxidation of the copper and enhance the electrical conductivity of the coating as a whole. That is, a synergistic effect is provided through the coating of the machine housing with both a copper and a graphite material.

EXAMPLE I The interior of a machine cover made up of Noryl (a moldable modified polyphenylene oxide marketed by General Electric Company) and molded into a console cover for an IBM Mag Card II was first cleaned with a solvent to remove contamination such as mold releases and dirt. To the cleaned and dryed interior surface was applied, through spray painting techniques, an Electrodag 435 (marketed by Acheson Colloids Company, Port Huron, Mich.) copper conductive coating diluted one to one with a commercial grade lacquer thinner. This first coating was applied in a sufficient amount to provide a 3 mil dried thickness and was then allowed to dry. The drying time to touch at ambient temperature is three to five minutes and can be overcoated in 15 minutes. The composition, obtained through various analysis techniques, in terms of percent solids and percent solvent of the copper conductive wet coating diluted one to one with a commercial grade lacquer thinner (such as AL-IOO, marketed by Austin Lacquer Company, Austin, Texas) is as follows:

l. 7r Solids 39-43% (by weight) a) 7: Copper 3l-337r (by weight) b) 7: Binder (thermoplastic resin) 8-l 1% (by weight) 2. 7i: Solvent 57-6l7z (by weight) a) Solvent Composition (7( of total solvent present) I lsopropyl alcohol and ethyl 3.2871 alcohol mixture 2) lsobutyl alcohol 19.26% 3) Methyl isobutyl ketone 5.58% 4) N-Butyl acetate 8.2l7r 5) Toluene 381371 6) P-Xylene and M-Xylene mixture 2.8271 7) O-Xylene l.97'7r 8) Three unidentified components 20.707:

(believed to be a mixture of Total 99.957:

isobutyl acetate and xylene derivatives) 2* Registered Trademark, Acheson Colloids Company 3. Binder Composition Thermoplastic ethyl cellulose resin.

To the copper coating following drying was applied an Electrodag ES 3376 (marketed by Acheson Colloids Company) coating which is a graphite pigmented water-dispersed resin.

The composition of the Electrodag ES337G, as determined by various analysis techniques, is set out below.

1. 7r Solids (by weight) 39-42% a) Carbon (in entire paint) 26-30% (by weight) b) 7r Binder (water soluble resin) l l-l57r (by weight) 2. 7: Solvent (by weight) 58-61% a) Solvent Composition (/1 of total solvent present) I Water 98% 2) Unidentified residual 2% solvent (believed to be Total I007:

a surfactant) 3. Binder Composition Vinyl acetate resin which upon drying and curing is a homopolymer of polyvinyl acetate.

The above coated housing provided good shielding from electromagnetic energy such as radar and other frequency radiation. It also provided good electrical continuity for discharge of electrostatic energy to ground.

EXAMPLE 2 A first coating of metallic copper pigment was applied as set out in Example 1. Within two to three days, electrical continuity became noticeably diminished due to oxidation of the copper. Good electromagnetic shielding was maintained though. Thus, the copper pigment alone would not maintain sufficient electrical conductivity for discharge of electrostatic energy over a desired time interval. Further, the copper coating when used alone had a tendency to flake. This flaking had an adverse effect on the card reading mechanism in the IBM Mag Card ll Console.

EXAMPLE 3 The graphite coating referred to in Example 1 was applied alone to the housing as set out in Example 1. This coating proved inadequate for purposes of both electromagnetic shielding and electrostatic grounding.

EXAMPLE 4 The two coating process referred to in Example 1 provided greater electromagnetic shielding and electrostatic grounding qualities than the sum of the one coating processes of Examples 2 and 3 over a desired useful life.

In summary, a conductive coating is applied to an article for providing electromagnetic and electrostatic energy shielding and grounding through essentially a two step coating process. The article is first coated with a sufficient amount of a metallic copper pigment dispersed in a thermoplastic resin binder to provide a pigment particle to particle contact upon drying. Thereafter, the copper pigment coating is coated with a sufficicnt amount of conductive graphite, made up of a graphite pigment dispersed in a resin, to prevent oxidation of the copper coating and enhance electrical conductivity.

While the invention has been particularly shown and described with reference to a particular embodiment, it will be understood by those skilled in the art that various changes in form and detail may be made without departing from the spirit and scope of the invention.

What is claimed is:

1. A process of applying a conductive coating to an article comprising:

a. coating said article with a metallic copper pig mented thermoplastic resin;

b. drying said copper pigmented coating;

c. coating said copper coating with a sufficient amount of a graphite pigmented water-dispersed resin to cover, and prevent oxidation of, said copper pigment, upon drying; and

d. drying said graphite pigmented coating.

2. a process according to claim 1 wherein said copper pigmented resin contains within the range of from about 25 to 40% by weight metallic copper pigment.

3. A process according to claim 1 wherein said graphite pigmented resin contains within the range of from about 20 to 40% by weight carbon as graphite.

4. A process according to claim 1 wherein said thermoplastic resin is ethyl cellulose.

5. A process according to claim 1 wherein said article is coated with a sufficient amount of said metallic copper pigmented thermoplastic resin to provide a pigment particle to particle contact upon drying.

6. A process of applying a conductive coating to an article for electromagnetic shielding electrostatic grounding purposes, said process comprising:

a. coating said article with a metallic copper pigmented thermoplastic resin containing within the range of from about 25 to 40% by weight metallic copper pigment;

b. drying said copper pigmented coating;

c. coating said copper coating with a sufficient amount of a graphite pigmented water-dispersed resin containing within the range of from about 20 to 40% by weight carbon as graphite to cover, and

prevent oxidation of, said copper pigment upon drying; and

d. drying said graphite pigmented coating.

7. A process according to claim 6 wherein said article is coated with a sufficient amount of said metallic copper pigmented thermoplastic resin to provide a pigment particle to particle contact upon drying.

8. A process according to claim 6 wherein said article is coated with a sufficient amount of said metallic cop per pigmented thermoplastic resin to provide a dried coating thickness of at least about 3 mils. 9. A process according to claim 8 wherein said copper coating is coated with a sufficient amount of said graphite pigmented water-dispersed resin to provide a dried graphite coating of at least about 1 mi].

10. A process according to claim 9 wherein said thermoplastic resin is ethyl cellulose.

11. A process according to claim 10 wherein said water-dispersed resin is comprised of vinyl acetate.

12. A process according to claim 9 wherein said copper pigmented resin contains within the range of from about 5 to 15% by weight thermoplastic resin binder.

13. A process according to claim 12 wherein said graphite pigmented resin contains within the range of from about 8 to 20% by weight water-dispersed resin 

1. A process of applying a conductive coating to an article comprising: a. coating said article with a metallic copper pigmented thermoplastic resin; b. drying said copper pigmented coating; c. coating said copper coating with a sufficient amount of a graphite pigmented water-dispersed resin to cover, and prevent oxidation of, said copper pigment, upon drying; and d. drying said graphite pigmented coating.
 2. a process according to claim 1 wherein said copper pigmented resin contains within the range of from about 25 to 40% by weight metallic copper pigment.
 3. A process according to claim 1 wherein said graphite pigmented resin contains within the range of from about 20 to 40% by weight carbon as graphite.
 4. A process according to claim 1 wherein said thermoplastic resin is ethyl cellulose.
 5. A PROCESS ACCORDING TO CLAIM 1 WHEREIN SAID ARTICLE IS COATED WITH A SUFFICIENT AMOUNT OF SAID METALLIC COPPER PIGMENTED THERMOPLASTIC RESIN TO PROVIDE A PIGMENT PARTICLE TO PARTICLE CONTACT UPON DRYING.
 6. A process of applying a conductive coating to an article for electromagnetic shielding electrostatic grounding purposes, said process comprising: a. coating said article with a metallic copper pigmented thermoplastic resin containing within the range of from about 25 to 40% by weight metallic copper pigment; b. drying said copper pigmented coating; c. coating said copper coating with a sufficient amount of a graphite pigmented water-dispersed resin containing within the range of from about 20 to 40% by weight carbon as graphite to cover, and prevent oxidation of, said copper pigment upon drying; and d. drying said graphite pigmented coating.
 7. A process according to claim 6 wherein said article is coated with a sufficient amount of said metallic copper pigmented thermoplastic resin to provide a pigment particle to particle contact upon drying.
 8. A process according to claim 6 wherein said article is coated with a sufficient amount of said metallic copper pigmented thermoplastic resin to provide a dried coating thickness of at least about 3 mils.
 9. A process according to claim 8 wherein said copper coating is coated with a sufficient amount of said graphite pigmented water-dispersed resin to provide a dried graphite coating of at least about 1 mil.
 10. A process according to claim 9 wherein said thermoplastic resin is ethyl celluLose.
 11. A process according to claim 10 wherein said water-dispersed resin is comprised of vinyl acetate.
 12. A process according to claim 9 wherein said copper pigmented resin contains within the range of from about 5 to 15% by weight thermoplastic resin binder.
 13. A process according to claim 12 wherein said graphite pigmented resin contains within the range of from about 8 to 20% by weight water-dispersed resin binder. 